Mass spectrometer for gases at low pressures



April 16, 1963 J. PEPER 3,086,111

MASS SPECTROMETER FOR GASES AT LOW PRESSUR EIS Filed May 10, 1960 2Sheets-Sheet 1 lNVENTOR JAN PEPER BY W r AGENT April 6, 1963 J. PEPER3,086,111

MASS SPECTROMETER FOR GASES AT LOW PRESSURES Filed May 10, 1960 2Sheets-Sheet 2 United States Patent GASES AT This invention relates tomass spectrometers for gases at low pressures, in which an electron beamof small cross-sectional area is projected in the direction of ahomogeneous static magnetic field, which beam produces ionization of thegas to be examined, and in which provision is also made of ahigh-frequency electric field at right angles to the magnetic field, thehigh-frequency field being homogenized by electrodes arranged parallelto the direction of the magnetic field.

Mass spectrometers of the above-mentioned known type, which may dispensewith homogenization of the high-frequency field and are referred to asomegatrons, are capable of determining the composition of a gas at verylow pressures, since only ions of a certain massto-charge ratio describespirally increasing paths at a certain frequency. By varying thefrequency of the high-frequency field, the mass spectrum is scanned andfrom the collected ion current as a function of the frequency theintensity of the masses is determined.

It has, however, been found that many types of omegatrons have a lowersensitivity and/ or resolving power than would be expected ontheoretical grounds. One of the reasons therefor is that, owing to thevarious voltages set up at the electrodes enclosing the rectangularionization chamber, fields are produced in this chamber due to which theresonant frequency for a certain kind of ions is not the same throughoutthe entire chamber.

It is an object of the present invention to provide a mass spectrometerin which the above-mentioned disadvantages are at least substantiallyobviated.

In a mass spectrometer for gases at low pressures, in which an electronbeam of small cross-sectional area is produced in the direction of ahomogeneous static magnetic field, which beam brings about ionization ofthe gas to be examined, while provision is also made of a high-frequencyelectric field at right angles to the magnetic field, thishigh-frequency field being homogenized by electrodes arranged parallelto the direction of the magnetic field, according to the invention thechamber in which the gas to be examined is ionized by the electron beamis bounded by electrodes the boundaries of which comprise the lines ofintersection of planes with a surface of revolution of three sheets,which surface is obtained by revolution of two conjugate hyperbolaeabout the axis of one of them, which coincides with the axis of theelectron beam, while the electrodes the boundary lines of which aredetermined by the two outer sheets of the surface of revolution are at asmall positive voltage relative to the electrodes the boundary lines ofwhich are determined by the central sheet of the surface of revolution.Preferably, the asymptotes to the hyperbolae from which the surface ofrevolution is derived make an angle with the axis of revolution suchthat the tangent is equal to /2.

The electrode configuration in accordance with the invention ensuresthat the radial field strength produced by the small voltage differencebetween the centre elec trodes and the outer electroeds is proportionalto the distance from the axis. Hence, the resonant frequency is constantfor all ions throughout the entire chamber.

The voltage difference between the inner and outer electrodes isnecessary to prevent the resonant ions from 3,986,111 Patented Apr. 16,19 63 drifting too far in the axial direction, so that they would nolonger contribute to the desired ion current.

In order that the invention may readily be carried into effect, anembodiment will now be described in detail, by way of example, withreference to the accompanying drawings, vin which:

FIG. 1 shows the conjugate hyperbolae from which the surface ofrevolution is derived,

FIG. 2 shows the electrodes, and

FIG. 3 shows a circuit arrangement comprising an omegatron in accordancewith the invention partly in cross-section.

' FIG. 1 shows co-ordinate axes z and r at right angles to one another,in which two branches I, II, resp., III, IV of two conjugate hyperbolaeare drawn between two asymptotes A and A The co-ordinate z representsthe axial co-ordinates in the electrode system and the co-ordinate rindicates the distance from the axis. The surface of revolutiondetermining the boundary lines of the electrodes is obtained byrevolving the branches of the hyperbolae about the z-axis. Thus, threesheets are obtained, namely a centre sheet the axial section of which isgiven by the branches I and II, (the surfaces of revolution of branchesI and II coincide) and two outer sheets the axial sections of which aregiven by the branches III and IV. The angle on made by the asymptotes Aand A with the z-axis is such that tan oa=\/2. It can be proved that bythis form the condition of proportionality between the radial fieldstrength and the distance from the axis is exactly satisfied.

FIG. 2 is a perspective view of the electrodes bounding the ionizationchamber. Electrodes 1 are obtained by intersection of planes parallel tothe z-axis with the outer sheet corresponding with the branch IV and theelectrodes 3 are obtained by intersection of these planes with the sheetcorresponding with the branch III. For the sake of clearness, only partof the electrodes 1 are shown partially broken away. Electrodes 2 areobtained by intersection of the sheet corresponding with the branches Iand II with the planes extending parallel to the z-axis. The electrodes1 and 3 associated with the same plane are connected to one anotherelectrically, as is shown in the drawing. The electrodes 2 which aresplit in two are also connected to one another electrically.

In the mass spectrometer shown in FIG. 3, the electrodes .1, 2 and 3 areshown partly in elevation and the remaining electrodes in cross-section.The remaining electrodes comprise a thermionic cathode 4 arranged withina screen electrode 5. A control electrode 6 may be used for maintainingthe electron current constant. Two screening electrodes 7 and 8 areprovided, similarly to the electrodes 5 and 6 and the centre electrodes1 and 3, are provided with apertures for the passage of an electronbeam. A collector for the electron beam is designated 9. The electrode 5and the cathode 4 are at a negative potential of about v. with respectto the earthed electrodes 7 and 8, while the electrode 6 is at apositive potential of about 10 v. with respect to the electrode 7. Theelectrodes 1 and 3 on the one hand and the electrode 2 on the other areconnected to potentiometers 10 and 11 respectively, which both comprisea number of resistors. A positive voltage -of from 0.3 to 0.4 v. withrespect to earth is produced across the potentiometer it), while thepotentiometer 11 is at earth potential. A high-frequency voltage havingan effective value of about 1 v. across terminals 12 is applied throughthe two potentiometers 10 and 11 to the electrodes. For the sake ofclarity, not all the connec-. tions to the electrodes are shown. Theions which under the influence of the fields describe paths ofincreasing radius, are collected by the electrodes 2 and this ion flowis measured by means of a sensitive direct-current amplifier 13connected between the potentiometer 11 and earth. The magnetic field isindicated by arrows H.

What is claimed is:

1. A mass spectrometer for gases at low pressures comprising means togenerate and project an electron beam of given cross-sectional area in agiven direction, means to generate a homogeneous static magnetic fieldin a region of gas through which the electron beam passes and ionizesthe gas to be examined, means to generate a high-frequency electricfield which is at right angles to the magnetic field and is homogenizedby electrodes positioned parallel to the direction of the magneticfield, said region being bounded by electrodes the boundaries of whichcomprise the lines of intersection of planes with a surface ofrevolution of thre sheets obtained by revolving two conjugate hyperbolaeabout the axis of one of them, which coincides with the axis of theelectron equal to V5.

References Cited in the file of this patent UNITED STATES PATENTS2,829,260 Donner et al. Apr. 1, 1958 FOREIGN PATENTS 773,689 GreatBritain May 1, 1957

1. A MASS SPECTROMETER FOR GASES AT LOW PRESSURES COMPRISING MEANS TOGENERATE AND PROJECT AN ELECTRON BEAM OF GIVEN CROSS-SECTIONAL AREA IN AGIVEN DIRECTION, MEANS TO GENERATE A HOMOGENEOUS STATIC MAGNETIC FIELDIN A REGION OF GAS THROUGH WHICH THE ELECTRON BEAM PASSES AND IONIZESTHE GAS TO BE EXAMINED, MEANS TO GENERATE A HIGH-FREQUENCY ELECTRICFIELD WHICH IS AT RIGHT ANGLES TO THE MAGNETIC FIELD AND IS HOMOGENIZEDBY ELECTRODES POSITIONED PARALLEL TO THE DIRECTION OF THE MAGNETICFIELD, SAID REGION BEING BOUNDED BY ELECTRODES THE BOUNDARIES OF WHICHCOMPRISE THE LINES OF INTERSECTION OF PLANES WITH A SURFACE OFREVOLUTION OF THREE SHEETS OBTAINED BY REVOLVING TWO CONJUGATEHYPERBOLAE ABOUT THE AXIS OF ONE OF THEM, WHICH COINCIDES WITH THE AXISOF THE ELECTRON BEAM, THE ELECTRODES THE BOUNDARY LINES OF WHICH AREDETERMINED BY THE TWO OUTER SHEETS OF THE SURFACE OF REVOLUTION BEING ATA LOW POSITIVE POTENTIAL WITH RESPECT TO THE ELECTRODES THE BOUNDARIESLINES OF WHICH ARE DETERMINED BY THE CENTRE SHEET OF THE SURFACE OFREVOLUTION.